The present disclosure relates to fluid-to-fluid bundled tube heat exchangers and methods of manufacture. More particularly, the present disclosure relates to bundled capillary tube heat exchangers useful in cardiopulmonary bypass circuits and including a knitted or woven tube mat symmetrically wound about a mandrel, and related methods of manufacture.
Fluid-to-fluid heat exchangers are used in many different industries, and are typically constructed in conjunction with the intended end use. For example, a heat exchanger is an important component of an extracorporeal or cardiopulmonary bypass circuit. As a point of reference, an extracorporeal blood circuit is commonly used during cardiopulmonary bypass (i.e., a heart-lung bypass machine) to withdraw blood from the venous portion of the patient's circulation system and return the blood to the arterial portion. The extracorporeal blood circuit generally includes a venous line, a venous blood reservoir, a blood pump, an oxygenator, a heat exchanger, an arterial line, and blood transporting tubing, ports, and connection pieces interconnecting the components. The oxygenator raises the oxygen content and reduces the carbon dioxide content of the blood arriving from the patient. The heat exchanger regulates a temperature of the extracorporeal blood as desired. For example, the heat exchanger can be located upstream of the oxygenator and operated to cool the blood arriving from the patient prior to oxygenation; alternatively, the heat exchanger can be operated to warm the extracorporeal blood.
Regardless of the direction of heat transfer between the heat exchanger and the patient's blood, extracorporeal blood circuit heat exchangers are generally made of a multiplicity of metal or plastic tubes; a suitable heat transfer fluid, such as water, is pumped the through the tube lumens while the blood flows about the tube exteriors. The heat exchange fluid can be heated or cooled (relative to a temperature of the blood). As blood contacts the tubes, heat transfer occurs between the blood and the heat exchange fluid in an intended direction. Alternatively, blood flow can be through the tube lumens, with the heat exchange fluid flowing about the tube exteriors.
So as to have minimal impact on the circuit's prime volume, the extracorporeal heat exchanger is desirably as small as possible while still providing high heat exchange efficiency. To meet these requirements, extracorporeal heat exchanger tubes are micro-diameter or fiber-like (e.g., outer diameter of 0.05 inches or less), and are oftentimes referred to as capillary tubes. The heat exchange fluid is fluidly isolated from blood of the extracorporeal circuit by a wall thickness of the capillary tubes, keeping the fluids separate but allowing the transfer of heat from one fluid to the other.
A common capillary tube format pre-assembles a large number of the micro-diameter tubes into a mat. The tubes are held together with threads forming the warp of the mat. The tube mat is then wrapped in some fashion to form a capillary tube bundle useful for heat exchanger applications. Typically, the mat is wrapped or rolled around a core or mandrel. As the mat is continuously wound about the mandrel, the mat winds onto itself, resulting in series of radially increasing layers. In light of this well-accepted heat exchanger bundling practice, the capillary tubes of the mat are “biased” so that the tubes are not parallel with a width of the mat. The purpose of the bias angle in the capillary tubes is to prevent the capillaries of subsequent layers from nesting in the gaps between the capillary tubes of an immediately preceding layer as the mat is wound onto itself. The tube angle orientation alternates from layer-to-layer. This arrangement is reflected in FIG. 2C described in greater detail below, and generally entails the tubes of an outer layer and the tubes of an immediately underlying layer being arranged at a crossing angle relative to each other. This crossing angle effect is commonly achieved by forming the bundled construction from a single, pre-made mat having two layers arranged with the alternating tube orientation, or by arranging two separately made mats at opposing tube angles and then wrapping the two mats as a pair.
Conventionally, when using a cylindrical mandrel or core to form the capillary tube bundle, an edge of each of a pair of mat layers with alternating tube orientations are attached to one side of the mandrel and then rolled. While viable, this technique inherently results in an asymmetrical arrangement of the bundled mat (and thus of the mat tubes) relative to an axis of the mandrel. This asymmetry, in turn, may undesirably result in uneven flow distribution through the resultant heat exchanger. Further, because heat exchanger tube mats are commonly provided as dual layer or dual ply structures, an edge of the inner layer is attached to the cylindrical mandrel and then the dual layer mat is wrapped about the mandrel. Not only does this approach create the asymmetrical bundle construction described above, but has the further inherent drawback of the outer layer taking a longer path than the inner layer when wrapping about a curved surface. This inherent constraint limits a curvature of the wrap and thus how small the device can be made. Along these same lines, because the outer layer will take a longer path than the inner layer (when being wrapped about a cylindrical mandrel), if the inner and outer mat layers have the same length when flat, after being wrapped around the mandrel, the outer layer ends much sooner than the inner layer. This undesirable difference accumulates as additional layers are added. Even further, the tension required when rolling the mat is different for the outer layer as compared to the inner layer.
Rolled mat heat exchangers are widely regarded as uniquely providing the small footprint and high heat exchange efficiency requirements of extracorporeal circuits, at an economically viable cost. Any improvements to rolled mat heat exchanger performance and/or techniques for manufacturing a roiled mat heat exchanger will be well-received.